The present invention is directed to an apparatus for filtering noise from a periodic signal. In its preferred embodiment, the present invention is configured for filtering noise from an input clock signal for driving the system clock of an electronic circuit.
Earlier filter circuits for input clock signals typically employ Schmitt trigger devices. Schmitt trigger devices effect a filtering function by "ignoring" noise depending upon the amplitude of the noise. However, if the amplitude of noise exceeds, for example, one-half the swing of the clock signal being filtered, a Schmitt trigger will likely pass the signal as a true signal.
The present invention facilitates tailoring of where in a periodic signal noise is to be filtered, and such filtering is effected regardless of the amplitude of the noise. The present invention effectively "stretches" a positive pulse and a negative pulse of a clock signal to "ignore" noise occurring during a predetermined time period so that no spurious triggering of the filter circuit to pass the clock signal is possible, regardless of how noisy the signal may be (i.e., regardless of how great the amplitude of the noise).